Precipitation hardenable austenitic steel



United States Patent 3,441,406 PRECIPITATION HASRDEIDJIABLE AUSTENITIC TEE Karl Bungardt and Wolfgang Spyra, Krefeld, Germany, assignors to Germany Edelstahlwerke Aktiengesellschaft, Krefeld, Germany No Drawing. Continuation of application Ser. No. 409,586, Nov. 6, 1964. This application Apr. 8, 1968, Ser. No. 721,120 Claims priority, application Germany, Nov. 14, 1963,

42,938 Int. Cl. 'C21d 7/14; C22c 39/20 US. Cl. 75-128 8 Claims This application is a continuation of Ser. No. 409,586, filed Nov. 6, 1964 and now abandoned.

The invention relates to austenitic steels of high mechanical strength particularly with a high 0.2-1imit and a high coefiicient of thermal expansion between 19 and 22.10- mm./mm. C.

Such alloys are required more particularly in the construction of machinery, especially where light metal alloys heated to temperatures of about 300 C. and steel abut. Known alloys of this kind have roughly the following composition:

Percent Carbon 0.40-1.0 Nickel 6.014.0 Manganese 3.0-14.0 Chromium Up to 4.0 Vanadium, which may be completely or partly replaced by like quantities of molybdenum and/or tungsten Up to 2.0 Silicon Up to 2.0

Iron and the impurities introduced in smelting- Remainder 0.2-limit kg./mm. 30-40 Tensile strength kg./mm. 6090 Elongation (l =d percent 30-70 Reduction of cross section at fracture do 60-80 It is desirable further to improve the tensile strengths of these steels because in the construction of engines for instance, the cylinder head bolts are required to have an 0.2-limit of at least 85 kg./mm. Such an increase in strength may be achieved by cold drawing and the 0.2- limit can be raised in this way to about 130 kg./mm. without any significant quantity of martensite being formed. This would reduce the coefiicient of thermal expansion.

However, the cold drawing of heavier sections has the drawback that the degree of deformation decreases from the surface zone to the centre. Consequently the tensile strength differs in different parts of the section. Another disadvantage of strengthening by cold Working is that the parts must be machined after they have been work hardened. In the production of bolts and screws this leads to difliculties. The following comparison illustrates the effect of a 19 and 30% cold deformation, related to cross section, after the solution treatment has been performed.

These data were determined in the case of two alloys of the following composition:

3,441,406 Patented Apr. 29, 1969 Alloy No. C Si Mn P S Cr M0 Ni V The following properties were measured:

30 mins.

water plus 30 mins. 30% cold 1,050 OJ defor- Alloy 1 water mation 0.2-limit, kgJmrn. 32 98 Tensile strength, kg./mrn. 74

Elongat on (ln=5dn) percent 66 20 Reduction in cross section, percent 69 60 Coefficient of expansionXlO- in mm./mm. 0.

between 20 and 200' C 20. 2 20.7

In the course of further investigations by the inventor in connection with the further development of the known alloy with a view to improving its tensile strength and its 0.2-limit without cold working, it was found that this can be done by alloying small quantities of tantalum and/or niobium as well as nitrogen with the steel. Tantalum-niobium contents between 0.1 and 2.0% and nitrogen contents between 0.01 and 0.2%, the latter depending upon the Mn-content, are already sufiicient. If the nitrogen content is to be raised beyond 0.1 to 0.2% the alloy must be molten under nitrogen at higher pressure. A steel of the above mentioned composition (alloy 1 or 2) was found to accept about 0.4% N when molten down in a nitrogen atmosphere at a pressure of 20 atm. and then cooled.

The steels modified according to the invention are then preferably not work hardened but subjected to the known process of precipitation hardening. If after having undergone a solution treatment between 1000 and 1300 C. followed by cooling in air, water or a like cooling medium, the steel is reheated to temperatures between 400 and 900 C., carbides and nitrides of the tantalum and niobium and of other special carbide and nitride forming metals which may be present in the alloy, such as vanadium, tungsten and molybdenum, will precipitate in the alloy structure. These precipitates increase the strength and the 0.2-limit of the steel and suppress the formation of coarse grain during the solution treatment which must be performed at high temperatures.

Apart from the desired improvement in strength the step proposed by the present invention also results in a considerable reduction in hardening below that required by other steels of this type, and the risk of decarburisation is correspondingly lessened. In the previously mentioned known alloys (alloy 2) 50 hours are needed at a hardening temperature of 700 C. in order to raise the 0.2-limit from 19 to 67 kg./mm. In the case of the proposed steel the 0.2-limit can be successful raised to 104 kg./mm. at a temperature of 650 C. in the course of 8 hours.

If it is desired that the proposed steel, when it has been cooled from the temperature of the solution treatment, should be as soft as possible for convenient machinability, then the rate of cooling from the temperature of the solution treatment must be faster than when cooling in air. The steel is therefore preferably quenched in oil or water or in some other medium that has a like effect, such as a sand bath. The further treatment of the proposed steel by hot shaping can be performed in the forged state. If the proposed steel-as will generally be the case-is intended for making bolts and screws, particularly with cylindrical heads, then it is advisable to perform the final solution and annealing treatment after the steel has been shaped, possibly with the application of a medium for preventing decarburisation, such as a brine bath, a protective gas, or a protective coating.

The subject matter of the present invention is therefore a precipitation hardenable austenitic steel of high strength and toughness, having a co-eflicient of thermal expansion of 19 to 22.10 mm./mm. C. when hardened, said steel being characterised by the following composition:

In view of the high coefficients of expansion that are required it has been found best to include as low as possible a silicon content, more particularly to limit the silicon content to a maximum of 0.3%

In the proposed composition of the steel the chromium content plays an essential part, since chromium-containing steels are less liable to decarburise.

On the other hand, the chromium content is subject to limitation because chromium reduces the coefiicient of expansion.

Steels having optimum properties are within the following range:

Percent Carbon 0.60-0.75 Nitrogen 0.05-0.08 Nickel 11.5-12.5 Manganese 4.5-5.5 Niobium and/or tantalum 0.2-0.5 Vanadium 0.9-1.2 Chromium 2.5-3.5 Silicon 0.3 Phosphorus 0.02 Sulphur 0.02 Iron Remainder The improved properties of the novel alloy (alloy 3) are illustrated by the following example:

Measurements reveal that the above nickel-managanese steel containing additions of tantalum and niobium as well as nitrogen, after having been suitably solution treated, can achieve an 0.2-limit exceeding kg./mm. and good toughness whilst possessing a coefficient of thermal expansion between 20 and 200 C. of about 20.10 mm./mm. C.

The novel steel can be forged between 1150 and 900 C. It is advisable to submit the steel as cast or after preliminary forging to a homogenising thermal treatment which may be performed at temperatures between 1000 and 1300 C., particularly between 1150 and 1250 C. The time required for this thermal treatment will generally be between 2 and 25 hours. In order to prevent excessive scaling, as will be necessary more especially in the case of small workpieces, this treatment may be performed in a protective gas atmosphere. The size of the austenite grain is determined by the final forging process. After having undergone the solution treatment between 1000 and 1300 C., preferably between 1150 and 1250 C., possibly in a protective medium for the prevention of decarburisation and scaling, the steel is quenched in water, oil or a medium having a similar quenching effect.

The precipitation hardening treatment is performed in the temperature region between 400 and 900 C., particularly between 550 and 750 C. The time required for maximum hardening depends upon the temperature and any possible previous work hardening to which the steel has been subjected and may be anything between a few minutes to several hundred hours. Hardening can also be achieved if the steel is slowly cooled from the temperature of the solution treatment to about 500 C. However, this type of temperature control is more difficult to perform than that of an isothermic heat treatment.

The mechanical properties of this novel steel after maximum precipitation hardening can also be further improved by work hardening after the solution treatment has taken place. Degrees of cold deformation between 5 and 30% have proved to be best. The scale formed during the precipitation hardening process can be removed by pickling, for instance in a 20% sulphuric acid at 50 to 60 C.

We claim:

1. A steel having a coefllcient of expansion of between 19x10 rum/mm. C. and 22x10 mm./mm. C., and a 0.2% elastic limit of at least 85 kg./mm. said steel consisting essentially of:

Percent Carbon 0.60-0.75 Nitrogen 0.05-0.08 Nickel 11.5-12.5 Manganese 4.5-5.5 Niobium and/or tantalum 0.2-0.5 Vanadium 0.9-l.2 Chromium 2.5-3.5 Silicon 0.3 Phosphorus 0.02 Sulphur 0.02 Iron Remainder 2. The method which comprises providing a steel having the following composition: .60-.75% carbon; .05- .08% nitrogen; 1l.5-12.5% nickel; 4.5-5.5% manganese;

5 0.2.5% niobium and/ or tantalum; .91.2% vanadium; 25-35% chromium; less than .3% silicon; less than .02% phosphorus; less than .02% sulphur; remainder iron; homogenizing the said steel for 2-25 hours between 1,000 and 1,300 degrees C.; and finish forging the homogenized steel at temperatures between 1,150 and 900 degrees C.

3. The method according to claim 2, in which the steel is homogenized at between 1150 and 1250 C.

4. The method according to claim 2, in which the steel after casting is preforged and then subjected to the homogenizing treatment.

5. The method according to claim 2, in which the steel after having undergone said homogenizing treatment is quenched and is precipitation hardened in the temperature range between 400 and 900 C.

6. The method according to claim 5, in which the steel after having undergone the homogenizing (solution) treatment is cooled and cold worked to deform it by 5 to 30% and is precipitation hardened.

7. The method according to claim 5, in which the steel having undergone the homogenizing treatment is cooled and cold worked to deform it by 10 to 20% and is precipitation hardened.

6 8. The method according to claim 2, in which the steel after having undergone said homogenizing treatment is quenched and is precipitation hardened in the temperature range between 550 and 750 C.

References Cited UNITED STATES PATENTS 3,163,526 12/1964 Harpster 75-128 2,334,816 11/1943 Frevert et a1. 75l28 2,739,057 3/1956 Payson 75123 2,751,291 6/1956 Carter et a1 75l28.6 3,062,692 11/1962 Manganello et a1. 75l28.6 FOREIGN PATENTS 741,053 11/1955 Great Britain. 152,371 10/ 1920 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

US. Cl. X.R. 14812, 12.3, 12.4

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 2.441.406 Dated Avril 29. 1969 Patent No.

Invencor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the specification, "Germany Edelstehlwerke Aktiengesellschaft" should read --Deutsche Edelscahlwerke Akt1engesellschaft--.

31cm AND SEALED FEB I '7 I970 4mm Anew WILLIAM L SGH YLER, IR. EdvudlLFletchel'Jh Gomissioner of Patents Atteating Officer 

1. A STEEL HAVING A COEFFICIENT OF EXPANSION OF BETWEEN 19X10**6 MM./MM. *C. AND 22X10**6 MM./MM. *C., AND A 0.2% ELASTIC LIMIT OF AT LEAST 85 KG./MM.2, SAID STEEL CONSISTING ESSENTIALLY OF: 